This invention relates generally to optical receiver systems and particularly to a narrow angle, narrow bandwidth optical receiver system for indicating signals of interest within a predetermined area illuminated by a laser beam having a known wavelength.
In a reconnaissance system it is often desirable to detect and track objects of interest or targets disposed and submersed within a predetermined area. For example, an aircraft such as a helicopter would hover or traverse an area above a water body while directing a laser beam from the aircraft into the water body for search and detection of submersed objects of interest. Light rays reflected from the illuminated area, including rays both from a detected object and unwanted reflections such as sun rays are all radiated back toward the aircraft which employs an optical receiver system designed to receive the laser generated light reflections and to filter out undesirable reflections such as those generated by the sun. One prior art method used in the detection of such reflected rays requires the utilization of a relatively large diameter interference filter array positioned at the input to the optical system for transmitting the desired radiation and inhibiting transmission of background noise radiation such as sun light. A correspondingly large collecting lens in conjunction with the filter provides maximum collection of optical energy with a minimal spectral bandpass in long range detection applications where light is at a premium. The large diameter filter, however, is usually constructed with a narrow bandpass of 25 angstroms (A) or less. A good quality, large optical filter array is relatively expensive and difficult to construct. Additionally, after a period of time the array deteriorates from separation of the various bonded layers.